Page 248 - Physics Coursebook 2015 (A level)

Cambridge International AS Level Physics
Summary
■■ The α-particle scattering experiment provides evidence for the existence of a small, massive and positively charged nucleus at the centre of the atom.
■■ Most of the mass of an atom is concentrated in its nucleus.
■■ The nucleus consists of protons and neutrons, and is surrounded by a cloud of electrons.
■■ The number of protons and neutrons in the nucleus of an atom is called its nucleon number A.
■■ The number of protons in the nucleus of an atom is called its proton number (or atomic number) Z.
■■ Hadrons (e.g. the neutron) are particles that consist of quarks and hence are affected by the strong nuclear force. Leptons (e.g. the electron) are particles that are unaffected by the strong nuclear force.
■■ The weak interaction acts between quarks and is responsible for β-decay.
■■ Isotopes are nuclei of the same element with a different number of neutrons but the same number of protons.
■■ Different isotopes (or nuclides, if referring to the nucleus only) can be represented by the notation ZAX, where X is the chemical symbol for the element.
■■ There are three types of ionising radiation produced by radioactive substances: α-particles, β-particles and γ-rays.
■■ In radioactive decay, the following quantities are conserved: proton number, nucleon number and mass–energy.
■■ The most strongly ionising, and hence the least penetrating, is α-radiation. The least strongly ionising is γ-radiation.
■■ Because of their different charges, masses and speeds, the different types of radiation can be identified by the effect of an electric or magnetic field.
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End-of-chapter questions
1 Before Rutherford’s model, scientists believed that the atom was made up of negatively charged
electrons embedded in a ‘plum pudding’ of positive charge that was spread throughout the atom.
Explain how the α-particle scattering experiment proved that this old model of the atom was
incorrect. [3]
2 A nucleus of strontium has a nucleon number of 90 and a proton number of 38. Describe the
structure of this strontium nucleus. [1]
3 State the changes that take place in a nucleus when it emits an α-particle and then two β−-particles. [5]
4 The nuclide of iodine with a nucleon number of 131 and a proton number 53 emits a β−-particle.
Write a nuclear equation for this decay. [3]
5 An isotope of carbon 164C emits a β−-particle and changes into an isotope of nitrogen (N).
a What are β−-particles? [1]
b Write a nuclear decay equation for the decay. [2]
c Draw a graph with the y-axis representing nucleon numbers between 10 and 16 and the x-axis
representing proton numbers between 4 and 10. On your graph mark:
i t h e i s o t o p e 1 64 C [2]
ii the daughter nucleus produced in the decay. [1]